休眠XML映射：懒惰错误或提取选择？ [英] Hibernate XML Mapping: Lazy False or Fetch Select?

问题描述

我有一个简单的问题。

 <多对一名称=雇员
 class我的项目中找到了这个Hibernate配置：
 =com.myapp.Employee
 cascade =merge
 lazy =false
 fetch =select> 
< column name =employee_id
 sql-type =bigint
 not-null =true/> 
< /多对一> 
  

不 fetch =select表示延迟加载所有基于获取策略的收藏和实体。但是通过写 lazy =false表示不要延迟加载。所以上面的配置说：禁用延迟加载，启用延迟加载。实际上，这意味着该属性被延迟加载？

所以我可以缩短配置为：

<$ p $ <多对一名称=员工
class =com.myapp.Employee
cascade =merge
fetch =select >
< column name =employee_id
sql-type =bigint
not-null =true/>
< /多对一>


但是不是默认模式？所以实际上，我可以声明相同的配置：

 <多对一名称=雇员
 class =com.myapp.Employee
 cascade =merge> 
< column name =employee_id
 sql-type =bigint
 not-null =true/> 
< /多对一> 
  

我是否正确？错误？想法？谢谢

如果我想启用延迟加载，我必须添加 lazy =true并删除 lazy =false？

解决方案

我认为抓取模式和抓取时间是不完全重叠的概念。

Lazy =true | false控制关联是否被急切或按需加载。

fetch =select | subselect | join | batch控制该实体或集合在需要加载时如何加载。

所以，要回答你的问题，有 fetch =select表示：

第二个SELECT用于检索关联的实体或集合，除非您通过指定lazy =false来显式禁用延迟获取，否则只有在访问该协会。 （ http：//docs.jboss。 org / hibernate / core / 3.3 / reference / en / html / performance.html＃performance-fetching ）

并不意味着懒惰加载被禁用！这是由 lazy =true | false标志控制的。
使用 lazy =true和 fetch =select Hibernate会延迟加载集合，加载一个选择。如果你设置了 lazy =false，同样的选择将被执行，不同之处在于它会被急切地执行。希望这是有道理的。

看看这里： http://community.jboss.org/wiki/AShortPrimerOnFetchingStrategies

I have a simple question. I found this Hibernate config on our project:

<many-to-one name="employee" 
    class="com.myapp.Employee"
    cascade="merge"
    lazy="false"
    fetch="select">
<column name="employee_id"
     sql-type="bigint" 
     not-null="true"/>
</many-to-one>

Doesn't fetch="select" mean "Lazy load all the collections and entities" based on Fetching Strategies. But by writing lazy="false" mean do not lazy load. So the config above says: "Disable lazy loading. Enable lazy loading." In effect, this means the property is lazy loaded?

So I could shorten that config as:

<many-to-one name="employee" 
    class="com.myapp.Employee"
    cascade="merge"
    fetch="select">
<column name="employee_id"
     sql-type="bigint" 
     not-null="true"/>
</many-to-one>

But isn't fetch="select" the default mode? So in effect, I can declare the same config as:

<many-to-one name="employee" 
    class="com.myapp.Employee"
    cascade="merge">
<column name="employee_id"
     sql-type="bigint" 
     not-null="true"/>
</many-to-one>

Am I correct? Wrong? Ideas? Thanks

If I want to enable lazy loading, I must add lazy="true" and remove lazy="false"?

解决方案

I think Fetch Mode and time of fetch are concepts that don't totally overlap.

Lazy="true|false" controls whether an association is loaded eagerly or on demand.

fetch="select|subselect|join|batch" controls how is that entity or collection loaded, when it's required to be loaded.

So, to answer your question, having fetch="select" means:

"a second SELECT is used to retrieve the associated entity or collection. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only be executed when you access the association." (http://docs.jboss.org/hibernate/core/3.3/reference/en/html/performance.html#performance-fetching)

This doesn't mean that lazy loading is disabled! That's controlled by the lazy="true|false" flag. With lazy="true" and fetch="select" Hibernate will lazy load the collection and will load it with a select. If you set lazy="false", the same select will be executed, the difference will be that it will get executed eagerly. Hope this makes sense.

Have a look here as well: http://community.jboss.org/wiki/AShortPrimerOnFetchingStrategies

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